In a cellular communication system, multiple antennas at a base station (BS) and multiple antennas at one or more user terminals (UTs) served by the BS allow one or more independent data streams to be transmitted from the BS to the UT(s) over the same time-frequency interval with precoding or beamforming. If more than one stream is transmitted, the transmission technique that makes this possible is referred to as spatial multiplexing. In general, spatial multiplexing is a multiple-input, multiple-output (MIMO) transmission technique that uses the different “paths” or channels that exist between the multiple antennas at the BS and the multiple antennas at the one or more UTs to spatially multiplex the independent data streams over the same time-frequency interval. When one UT is served two or more independent data streams by the BS over the same time-frequency interval, the system is said to be performing single-user MIMO (SU-MIMO), and when multiple UTs are each served one or more independent data streams by the BS over the same time-frequency interval, the system is said to be performing multi-user MIMO (MU-MIMO).
The number of independent data streams that can be transmitted over the same time-frequency interval can be shown to be limited by the lesser of the number of antennas at the BS and the total number of antennas at the one or more UTs. A further limitation on the number of independent data streams that can be transmitted over the same time-frequency interval results from interference between the independent data streams or what is referred to as inter-user interference in the MU-MIMO context.
In T. L. Marzetta, “Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas,” IEEE Transactions on Wireless Communications, vol. 9, no. 11, pp. 3590-3600, November 2010 [Marzetta], a concept referred to as “massive MIMO” was introduced. In general terms, massive MIMO refers to a communication system that has a large number of antennas available at the BS. The many antennas are used to reduce interference between independent data streams by further focusing the energy of each independent data stream into ever-narrower regions of space or beams. This is done by appropriately shaping the independent data streams so that the wave fronts emitted by the available antennas for each of the independent data streams add up constructively at the location of the UT intended to receive the independent data stream and destructively everywhere else (or at least everywhere else where another data stream is intended to be received over the same time-frequency interval). The process of shaping the independent data streams at the BS is known as precoding. The many antennas can be further used to transmit a single data stream to a single UT. In this case, the many antennas are similarly used to focus the energy of the data stream into a narrow region of space or beam, but through a specific precoding shaping process referred to as beamforming.
Because massive MIMO needs a large number of antennas at the BS to support a large number of streams and users, it also means that it generates narrow beams due to precoding or beamforming with the large number of antennas. The narrow beams can result in large equivalent isotropically radiated power (EIRP) in the respective direction each beam is focused. EIRP is the amount of power that a theoretical isotropic antenna (i.e., an antenna that radiates its power uniformly in all directions) would emit to produce the power measured in a particular direction. The Federal Communications Commission (FCC) and other regulatory bodies often place fixed constraints on the maximum EIRP that communication systems can emit in any given direction to limit interference and potential harmful health effects that high radiated power levels can have on humans. For massive MIMO systems, such fixed constraints can be too restrictive because these systems typically communicate over the millimeter wave band where path losses can be (and often are) significant due to high-levels of absorption from atmospheric gases, rain, and/or foliage, and also due to more diffuse scattering.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.